Migrate fire user guide off datashader

[brendancol]

Reason or Problem

docs/source/user_guide/fire.ipynb uses datashader only for rendering (shade/stack/Images). There is no rasterization to replace here.

Proposal

• Remove the datashader imports.
• Replace shade/stack/Images with matplotlib.
• Update any prose cells that mention datashader.
• Re-execute the notebook so outputs and the preview image regenerate cleanly.

Done when

• The notebook runs top to bottom with no datashader import and produces the expected plots.

Part of removing datashader as a dependency; final packaging removal is tracked in #3450.